1. Field of the Invention
The present invention relates to an endoscope system for performing narrowband light observation using a complementary color type imaging device, and a light source device used in the endoscope system.
2. Description Related to the Prior Art
In a recent medical field, diagnosis and treatment using an endoscope system, having a light source device, an electronic endoscope, and a processor device, are widely performed. The light source device produces illumination light and applies the illumination light to the inside of a human body cavity. The electronic endoscope images the inside of the body cavity irradiated with the illumination light by an imaging device, and produces an imaging signal. The processor device applies image processing to the imaging signal produced by the electronic endoscope to produce an observation image to be displayed on a monitor.
As an observation method used in the endoscope system, there is known narrowband light observation using special light (narrowband light) having a narrow wavelength band as the illumination light, in addition to normal light observation using normal light (white light) having a wide wavelength band as the illumination light. The narrowband light observation, for example, can improve visibility of a blood vessel pattern in a superficial layer of a mucosa membrane, though the blood vessel pattern is easily buried in optical information obtained under irradiation with the white light. Therefore, the narrowband light observation allows focusing attention on superficial blood vessels of the blood vessel pattern, and diagnosing the stage of a disease, the depth of a lesion, and the like from the state of the superficial blood vessels.
The narrowband light observation uses two types of narrowband light absorbable by hemoglobin in blood, that is, blue narrowband light having a center wavelength in the vicinity of 415 nm and green narrowband light having a center wavelength in the vicinity of 540 nm. As an imaging method in the narrowband light observation, there are known a frame sequential method in which the blue narrowband light and the green narrowband light are alternately applied and a monochrome imaging device captures an image whenever each type of light is applied, and a simultaneous method in which the blue narrowband light and the green narrowband light are simultaneously applied and a simultaneous imaging device having a color filter captures an image (see U.S. Pat. No. 8,531,512 and US Patent Application Publication No. 2009/0141125). The simultaneous method is inferior in resolution to the frame sequential method, but has the advantages of preventing a blur in the image and structural simplicity of the endoscope system.
The simultaneous imaging device includes a primary color type imaging device having a primary color filter and a complementary color type imaging device having a complementary color filter. The complementary color type imaging device is highly sensitive as compared with the primary color type imaging device, and hence used in an endoscope system that places importance on sensitivity.
The U.S. Pat. No. 8,531,512 and the US Patent Application Publication No. 2009/0141125 disclose a complementary color type imaging device having four types of pixels of magenta (Mg), green (G), cyan (Cy), and yellow (Ye) in which the Mg pixels and the G pixels are alternately arranged in odd-number rows, and the Cy pixels and the Ye pixels are alternately arranged in even-number rows, such that the Mg pixel, the Cy pixel, the Mg pixel, the Ye pixel, . . . are arranged in this order in odd-number columns, and the G pixel, the Ye pixel, the G pixel, the Cy pixel . . . are arranged in this order in even-number columns. This color filter pattern is referred to as a complementary-color checkered-pattern color-difference line sequential method.
This complementary color type imaging device is driven by a field readout method in which pixel signals of two rows adjoining in a column direction are read out in a mixed (added) state in each of an odd-number field and an even-number field. Thus, the complementary color type imaging device outputs a mixed pixel signal (hereinafter called a first mixed pixel signal) of the Mg pixel and the Cy pixel, a mixed pixel signal (hereinafter called a second mixed pixel signal) of the G pixel and the Ye pixel, a mixed pixel signal (hereinafter called a third mixed pixel signal) of the Mg pixel and the Ye pixel, and a mixed pixel signal (hereinafter called a fourth mixed pixel signal) of the G pixel and the Cy pixel. The complementary-color checkered-pattern color-difference line sequential method has the advantage that the first to fourth mixed pixel signals are easily converted into a primary color signal (RGB signal) by a simple operation.
However, in the case of the narrowband light observation, the above complementary color type imaging device has a problem of mixture of the blue narrowband light and the green narrowband light. As for the blue narrowband light, for example, out of the above first to fourth mixed pixels, the first mixed pixel (Mg+Cy) is highly sensitive to the blue narrowband light, so it is conceivable to produce an image (superficial image) of the blue narrowband light using the first mixed pixel signal, but the first mixed pixel is highly sensitive to the green narrowband light too. As for the green narrowband light, on the other hand, since the second mixed pixel (G+Ye) is highly sensitive to the green narrowband light, it is conceivable to produce an image (middle to deep image) of the green narrowband light using the second mixed pixel signal, but the second mixed pixel is slightly sensitive to the blue narrowband light too.
The use of the complementary color type imaging device has the advantages of high sensitivity and easy producibility of the primary color signal in the normal light observation. However, in the narrowband light observation, the complementary color type imaging device is inferior in color separability of a blue narrowband light component and a green narrowband light component. The superficial image and the middle to deep image are mixed, and the superficial blood vessels have low contrast. Therefore, it is desirable to improve the color separability in the narrowband light observation using the complementary color type imaging device.